[sburck]

I have been doing a bit of thinking about machine design and one question that keeps coming up is how easily linear rails deform. Of course they are not all the same with there being a variety of sizes and configurations, however I'm really comparing linear rails to box/dovetail ways in general.

I see a lot of talk about how "These rails can take an axial load of up to xxxx kg, so they are more than beefy enough." What I'm assuming is that the rated loads for linear rails are related to mechanical failure. Now all materials deform, and the deformation is dependent on stress. Stress is force/cross sectional area (sorry if I sound like I am lecturing, I'm just trying to explain why I think linear rails may have problems related to deformation.) Now a load of X kg on a linear rail will produce a force on some components with small cross sectional area's, specifically the ball bearings and possibly the bolts used to fasten the rails to the machine base. The points of contact between the ball bearings and the rails/blocks are going to be very thin lines between the ball and the round groove cut into the rail. Now compare this to a box / dovetail way and you can see that the cross sectional area of contact for a linear rail at points is miniscule when compared to that of a dovetail / box way.

Is it appropriate to put rails on a bench top machine when you may be applying a significant force to the workpiece if you are say roughing 304ss. Couldn't these rail+block assemblies overall deflect by a few thou? Maybe its not worth fussing over because when roughing you leave room for finishing cuts so a few thou doesn't matter, and then when doing the finishing cut the forces on the work piece are not significant?

Just some thoughts. You see rails on a lot of routers, plasma cutters and 3d printers etc, but not so many milling machines.

[underthetire]

Linear rails or linear guides? And most of todays vmcs use linear guides. Your assuming only one bearing contact per side, which may or may not be the case in all rails. As far as deflection, when mounted correctly (bolt down to a flat scraped surface, and side clamped to a casting boss) I really can't tell much difference. You need to remember a box and especially a dovetail system requires clearance to be able to move the axis, otherwise it's simply a locked one piece design. The one advantage of box is vibration dampening in comparison to linear. Dovetail is pretty much gone now in most machines that I've seen. There are other advantages to liner as well, no or much less Waylube, higher positioning accuracy since there is no stick slip, and cost to manufacture the machine. I'm not for one or the other, it really depends on the machine and it's use. In general, I prefer guides on mills, and box on lathes. This is simply because guides don't usually like catching a hard jaw on a chuck. That tends to lift the slide up and wiping out guides.